Salinity

Dr. Pamela Gore
Georgia Perimeter College

Objectives

1. Be able to answer the quesiton "What is salinity?"
2. Be able to list the most abundant elements in sea water.
3. Explain how and why the sallinity of cold ocean water and warm ocean water differs.
4. Explain how the ocean became salty.
5. Explain how global warming would influence the salinity of the ocean.
6. Explain how a new ice age would affect the salinity of the ocean.

Salinity of sea water vs. fresh water

Nearly all water contains dissolved chemicals. Even rain water. These dissolved chemicals are called "salts".

The salinity of normal ocean water is about 35 parts per thousand, total dissolved solids. This is written as 35 ^o/_oo or 35 ppth. A salinity of 35 ^o/_oo is the same as 3.5%.

A salinity of 35 ^o/_oo means that there are 35 pounds of salt per 1000 pounds of sea water.

The symbol resembles a percentage sign (%), but percent means per hundred. But the symbol with two zeros on the bottom of the fraction (^o/_oo) means per thousand.

When 1 cubic foot of sea water evaporates, it leaves about 2.2 pounds of sea salt.

Freshwater has a salinity less than about 5 ppth (5 ^o/_oo). This is the same as 0.5%, or 5000 ppm (parts per million).

When 1 cubic foot of freshwater evaporates, it leaves less than an ounce of salt (0.01 pound of salt).

Variations in salinity

Salinity varies slightly from place to place around the world, and also varies somewhat with the seasons (affected by temperature and precipitation). The salinity of sea water ranges from about 30 to 40 ^o/_oo.

Warm temperatures and high evaporation rates (particularly in shallow seas), raise the salinity. Warmer waters will tend to have higher salinity than cooler waters.

Freshwater input (rivers, precipitation, or melting ice) lowers the salinity. In polar areas where glaciers and ice caps are melting, the sea water has lower salinity.

Salinity of water other than sea water and freshwater

Water of salinity between freshwater and seawater is called brackish. Brackish water can be found in estuaries.

Water with salinity greater than normal sea water is called hypersaline.

For example, the Great Salt Lake, Utah, and the Dead Sea are hypersaline. Hypersaline lakes tend to occur in arid areas. They are land-locked basins with no outlet for the waters to flow from the lake. The salts are concentrated as a result of evaporation. Evaporation is so extreme in some lakes (ephemeral lakes or playa lakes) that they dry up completely every few years, leaving a salt pan.

Brine shrimp can live in salinities up to 300 ppth (about 10 times the salinity of sea water).

How did the sea become salty?

Rain water falling on the land has a part in the weathering (breakdown) of rocks and the erosion (transport) of weathered rock fragments.

Rivers carry:

• Larger rock fragments as bed load (rolling along the bottom of the stream),
• Smaller rock fragments and mineral grains as suspended load (within the water column),
• Ions or dissolved "salts" from the chemical weathering of rocks and minerals as dissolved load.

Feldspars are the most abundant minerals in the Earth's crust. Through a type of chemical weathering called hydrolysis, feldspars are altered to form clays, and their ions (Ca, K, and Na) are released to be carried in solution by running water, eventually making their way to the sea.

The weathering of other minerals and rocks also provides salts to the sea. For example, the calcite in limestone dissolves in slightly acidic natural waters, yielding Ca and CO₃.

Rivers carry an estimated 4 billion tons of dissolved "salts" to the seas each year.

Some of these dissolved solids will be deposited as sediment, so yearly gains may roughly balance sediment deposition.

Salts have become concentrated in the sea (compared with freshwater) because the sun's heat causes the evaporation of water, leaving the salts behind.

The most abundant elements in sea water and fresh water

The particular salts found in freshwater, sea water, and saline lake waters tend to differ. The table below shows some of the dominant salts in freshwater and sea water. At least 72 chemical elements have been identified in sea water, most in extremely small amounts.

The chemistry of ocean water and freshwater is obviously very different. Evaporation serves to increase the salinity of sea water, but the chemicals do not all increase the same amount when compared with river water. For instance, silica is a significant constituent of river water, but not of sea water. Calcium and bicarbonate are also greatly reduced in sea water relative to river water.

Thinking questions:

Which are the dominant chemical constituents of freshwater?

Which are the dominant constituents of sea water?

Why are they different?

What is happening to all of the silica and calcium and bicarbonate that the rivers are delivering to the oceans?

Life in the sea affects composition of sea water

The dissolved chemicals in sea water and freshwater are very different from each other. Part of the explanation is the role played by marine organisms (plants, animals, and single-celled organisms).

Many marine organisms (molluscs, foraminifera, corals, crabs, lobsters, barnacles, sea urchins, etc.) extract calcium and bicarbonate from the sea to construct their shells or skeletons.

Other marine organisms require silica to construct their shells (diatoms, radiolarians, some sponges).

No known biological process removes sodium from the sea. Therefore, it is very abundant in sea water.

Salts in saline lakes

No two saline lakes are exactly the same. The particular salts found within them are a result of the types of bedrock in the surrounding area, climate, and other factors. The chemicals that dominate saline lakes tend to be Na, Mg, Ca, CO₃, SO₄, and Cl.

Unusual and economically important minerals form by the evaporation of some saline lakes. These minerals include:

• Gypsum
• Borax
• Natron
• Trona
• Glauberite
• Epsomite
• Uranium minerals
• Lithium-bearing clays

Assignment: Find the economic uses of each of these minerals.

Climatic change and salinity

The Earth's climate has fluctuated over time.

During the Cretaceous Period (65 to 140 million years ago), the Earth was much warmer. No glaciers existed, and sea levels were much higher than they are today, worldwide. In fact, the interior of North America was flooded, all across the Great Plains, from the Gulf of Mexico northward through Canada. Sea level was several hundred feet higher than it is today.

During the Ice Ages which recurred periodically thoughout the Late Cenozoic, particularly in the Pleistocene Epoch (10,000 years to about 2 million years ago), much of the world's water was tied up in continental ice sheets. Glaciers covered almost one-third of the land. Consequently, sea level was about 400 feet lower than it is today, and the continental shelves were exposed as dry land.

We are currently in an interglacial period, but we still have glaciers on the Earth.

If all of the glaciers on the Earth today melted, the seas would rise about 260 feet.

Thinking questions:

1. How would global warming affect the oceans' salinity?

2. How would the next Ice Age affect the oceans' salinity?